Presently, the dominating trend in the demand of copy machines with multifunctions is the preference of ratio-variable copy machines which are capable of performing enlargements and reductions.
Therefore, a device meeting the requirement of continuous ratio variations is needed. A typical optical system for such a slit illumination type ratio variable copy machine is illustrated in FIG. 7, and will be described below. That is, the original sheet placed on the contact glass to be copied is slit-illuminated by the illumination lamp upon starting of the copy machine an image is formed on a drum having a light-sensitive coating by components such as a first mirror, a second mirror, a third mirror, an image forming lens, and a fourth mirror.
In such an optical system, an illumination lamp and the first mirror are unitizingly fixed to a first carriage. The second mirror and the third mirror are fixed to a second carriage, while, along an elongate guide, the first carriage is driven at a certain speed V, and the second carriage is driven at a half speed thereof in the same direction, in such a manner that the object distance between the original sheet and the lens on the one hand and the image distance between the lens and the light-sensitive medium on the other hand are kept constant, thereby making it possible to carry out the slit-illumination scanning.
Accordingly, in the case where the copying ratio is not to be varied in this optical system, the first carriage is placed at the standard position which is indicated by the solid lines, and at the same time, the second mirror and the third mirror are associatively displaced in the direction of the scanning. In the case where an enlargement is to be carried out, the same component is displaced toward the left side to the position indicated by the dotted lines, in the case where a reduction copying is to be carried out, the second and third mirrors and the lens are displaced respectively to the left and right. In the case where the original sheet is placed centrally, the lens is displaced in the direction of the scanning in the case of a varied ratio copying. For example, the lens is displaced correspondingly with the ratio variation by means of a wire connected to a motor, while the second and third mirrors, the cam, the wire and the pulley are also displaced. Even in the case where the original sheet to be copied is scanned by a uniform illumination in the slit direction, depending on the incident angle of the image of the original sheet relative to the lens, the illumination on the light-sensitive medium is varied by the governing formula of the fourth power of cosine, and the illumination on the opposite end portions is lowered compared with the portion at the optical axis.
In the case of an equivalent copying ratio, the non-uniformness of the illumination on the light sensitive medium is corrected, usually by making the width of the opposite end portions of the slit in the direction of the scanning larger than that of the center, or by varying the illumination distribution of the lamp, with the width of the slit being kept intact.
On the other hand, in the case of a different copying ratio, the distance between the original sheet and the lens and the distance between the lens and the light sensitive medium are varied. As will be explained with respect to FIG. 2, the incident angle .THETA.m(x) of the light beam which has been reflected from an image position x on the original sheet of FIG. 2 and has passed through the center of the lens will be different from the incident angle .THETA.mb(x) of the light beam which has been reflected from the same point under the equivalent copying ratio. The non-uniformity of the illumination for different positions on the light sensitive medium can be calculated based on the following formula: ##EQU1##
Assuming the image height 0 (the central position of the slit) to be 100%, the examples of the non-uniformness of the illumination for different heights of the image and for different copying ratios will be described with reference to the graph of FIG. 1. In the case of enlarged copyings, the decrease is 7% even under an enlargement ratio of 1.41 and at an image height of 150 mm, while, in the case of reduction copyings, the increase in some cases is shown to be 20% even under the ratio of 0.5 and at an image height of 75 mm, with the result that a non-uniformness of illumination in the form of too bright illumination on the opposite end portions of the slit is produced in the case of a reduction copying.
As will be discussed in the simplified drawing of FIG. 3, if the illumination distribution on the original sheet is shown as in (a), then the illumination distribution (b) on the surface of the drum will be produced as indicated by 6 in the case of an equivalent copying ratio, will be produced as indicated by 5 in the case of a reduction copying ratio, and will be produced as indicated by 7 in the case of an enlargement copying ratio. In order to correct such a non-uniform illumination, it has been proposed that two light adjusting plates be placed in order to shield the light beams from the opposite sides. But in this method, the light beams from the opposite sides enter after having been shielded, and therefore, the light beams for the upper height of the image are shielded off in large amounts, resulting in that, in the case where the reduction ratio is large, considerable portions of the opposite edges of the paper are not copied.